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Abstract

Data from 946 Angus and 351 Simmental females were used to quantify relationships between
calfhood and early-life traits of females with fertility in the fust two breeding seasons, and with calf
production in the fITst, in three and in eleven years of life. These traits were used to develop pre·
diction equations for fertility using logistic regression methodology. For productive performance,
three methodologies were compared: least squares, ridge regression and principal components re·
gression.
For heifers of both breeds, fertility was associated with birth date, weaning weight ratio and
weaning-yearling relative growth rate (RGR). The only calfhood trait that consistently affected
fertility in the second breeding season was birth weight of the cow. Fertility of fITst parity
Simmentals was also associated with birth-yearling RGR. First calving season traits explained a
higher proportion of variation in second breeding season fertility than calfhood traits of the cow.
Among these traits, calving date, calving ease score and sex of the calf were the most important.
The combination of birth date and weaning-yearling RGR produced the best models to predict
heifer fertility for both breeds. Fertility in the second breeding season was best predicted for Angus
cows when calving date, calving ease score and sex of the calf were used as regressors. For
Simmental cows, fertility in the second breeding season was best predicted from birth weight of the
calf and calving ease score.